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Hubble finds double Einstein ring

This rare event can provide information about dark matter, dark energy, distant galaxies, and even the shape of the universe.
Provided by Hubble ESA, Garching, Germany
This is an image of gravitational lens system SDSSJ0946+1006 as photographed by Hubble Space Telescope's Advanced Camera for Surveys.
NASA/ESA/R. Gavazzi and T. Treu/SLACS team
January 10, 2008
NASA's Hubble Space Telescope has revealed a never-before-seen optical alignment in space: a pair of glowing rings, one nestled inside the other like a bull's eye pattern. The double-ring pattern is caused by the other complex bending of light from two distant galaxies directly behind a foreground massive galaxy, like three beads on a string.

More than just a novelty, this very rare phenomenon can offer insights into dark matter, dark energy, the nature of distant galaxies, and even the curvature of the universe.

The ring was found by an international team of astronomers led by Raphael Gavazzi and Tommaso Treu of the University of California, Santa Barbara. The discoveryis part of the ongoing Sloan Lens Advanced Camera for Surveys (SLACS) program. The team is reporting its results at the 211th meeting of the American Astronomical Society in Austin, Texas.

The phenomenon, called gravitational lensing, occurs when a massive galaxy in the foreground bends the light rays from a distant galaxy behind it, in much the same way as a magnifying glass would. When both galaxies are exactly lined up, the light forms a circle, called an "Einstein ring," around the foreground galaxy. If another background galaxy lies precisely on the same sightline, a second, larger ring will appear.

Because the odds of seeing such a special alignment are estimated to be one in 10,000, Tommaso says that they "hit the jackpot." The odds of seeing this phenomenon are less than winning two consecutive bets on a single number at Roulette.
This is a zoom onto the Hubble Space Telescope Advanced Camera for Surveys image of gravitational lens system SDSSJ0946+1006, showing two concentric partial ring-like structures after subtracting the glare of the central, foreground galaxy.
NASA/ESA/R. Gavazzi and T. Treu/SLACS team
"Such stunning cosmic coincidences reveal so much about nature. Dark matter is not hidden to lensing," adds Leonidas Moustakas of the Jet Propulsion Laboratory. "The elegance of this lens is trumped only by the secrets of nature that it reveals."

The massive foreground galaxy is almost perfectly aligned in the sky with two background galaxies at different distances. The foreground galaxy is 3 billion light-years away. The inner ring and outer ring are comprised of multiple images of two galaxies at a distance of 6 billion and approximately 11 billion light-years.

SLACS team member Adam Bolton of the University of Hawaii's Institute for Astronomy in Honolulu first identified the lens in the Sloan Digital Sky Survey (SDSS). "The original signature that led us to this discovery was a mere 500 photons (particles of light) hidden among 500,000 other photons in the SDSS spectrum of the foreground galaxy," comments Bolton.

"The twin rings were clearly visible in the Hubble image," adds Tommaso. "When I first saw it I said 'wow, this is insane!' I could not believe it!"

The distribution of dark matter in the foreground galaxies that is warping space to create the gravitational lens can be precisely mapped. Tommaso finds that the fall-off in density of the dark matter is similar to what is seen in spiral galaxies (as measured by the speed of a galaxy's rotation, which yields a value for the amount of dark matter pulling on it), though he emphasizes there is no physical reason to explain this relationship.

In addition, the geometry of the two Einstein rings allowed the team to measure the mass of the middle galaxy precisely to be a value of 1 billion solar masses. The team reports that this is the first measurement of the mass of a dwarf galaxy at cosmological distance (redshift of z=0.6).

A sample of several dozen double rings such as this one would offer a purely independent measure. The comparative radius of the rings could also be used to provide an independent measure of the curvature of space by gravity. This would help in determining the matter content of the universe and the properties of dark energy.

Observations of the cosmic microwave background (a relic from the Big Bang) favor flat geometry. A sample of 50 suitable double Einstein rings would be sufficient to measure the dark matter content of the universe and the equation of state of the dark energy (a measure of its pressure) to 10 percent precision. Other double Einstein rings could be found with wide-field space telescope sky surveys that are being proposed for the Joint Dark Energy Mission (JDEM) and recently recommended by the National Research Council.
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